The first book of Enoch says that 200 Watchers (angels) descended to earth and took wives (in the book of Genesis they are called sons of God). Then in the book of Enoch it is said that these 200 spirits should be taken into custody.
CHAPTER XII. 1. Before these things Enoch was hidden, and no one of the children of men knew where he was hidden, and where he abode, and what had become of him. 2. And his activities had to do with the Watchers, and his days were with the holy ones. 3. And I, Enoch was blessing the Lord of majesty and the King of the ages, and lo! the Watchers called me--Enoch the scribe--and said to me: 4. 'Enoch, thou scribe of righteousness, go, †declare† to the Watchers of the heaven who have left the high heaven, the holy eternal place, and have defiled themselves with women, and have done as the children of earth do, and have taken unto themselves wives: "Ye have wrought great destruction on the earth: 5. And ye shall have no peace nor forgiveness of sin: and inasmuch as †they† delight themselves in †their† children" ... CHAPTER XIII. 1. And Enoch went and said: 'Azâzêl, thou shalt have no peace: a severe sentence has gone forth against thee to put thee in bonds ... CHAPTER XVIII 13. I saw there seven stars like great burning mountains, and to me, when I inquired regarding them, 14. The angel said: 'This place is the end of heaven and earth: this has become a prison for the stars and the host of heaven. 15. And the stars which roll over the fire are they which have transgressed the commandment of the Lord in the beginning of their rising, because they did not come forth at their appointed times. 16. And He was wroth with them, and bound them till the time when their guilt should be consummated (even) ⌈for ten thousand years⌉.' CHAPTER XIX. 1. And Uriel said to me: 'Here shall stand the angels who have connected themselves with women, and their spirits assuming many different forms are defiling mankind and shall lead them astray into sacrificing to demons ⌈⌈as gods⌉⌉, (here shall they stand,) till ⌈⌈the day of⌉⌉ the great judgement in which they shall be judged till they are made an end of.
- that's what these spirits are all about in 1Petr 3:19, here we must to understand that the author of the Epistle of Peter has in view the plot of the antediluvian events according to the book of Enoch.